Nick Timothy is Director of the New Schools Network and a former Chief of Staff to Theresa May.

Those who cannot remember the past, so they say, are condemned to repeat it. And given that the debate about Britain’s membership of the European Union is the third great split in the history of the Conservative Party – after the repeal of the Corn Laws in 1846 and the indecision about tariff reform that led to defeat in 1906 – there are lessons for us today in those two schisms.

The first, and most obvious, is that when a party comes to believe that a single policy is more important than anything else – and when that policy is something upon which it disagrees – it can find itself out of power for a long time. In the general election of 1847, the year after Sir Robert Peel decided to repeal the Corn Laws in defiance of the protectionists who constituted most of his Party, the Conservatives still managed to win a majority. But Peel’s followers – who disagreed with the Party leadership that succeeded Peel about reinstating the Corn Laws – opted to put Lord John Russell’s Whigs into power rather than support their former colleagues. It was not for another 27 years – in 1874 – that the Conservatives won a majority in the House of Commons again.

In 1906, when the Unionist Government went to the country divided about whether to stick with free trade or to introduce tariff reforms – which under Joseph Chamberlain’s proposal would have seen free trade within the Empire and tariffs for goods from other countries – Balfour’s Conservatives were smashed by Henry Campbell Bannerman’s Liberals. They only found their way back to office when, following the Gallipoli disaster in 1916, a coalition was formed to lead Britain through the First World War.

So while the decision we face about our membership of the EU is of huge importance, Conservatives on both sides of the debate need to remember, regardless of Europe, we still need to fix the economy, continue reforming public services, and protect the country from terrorism. On all three of those challenges, the alternative to a Conservative government does not bear thinking about – yet a rancorous, bitter and lasting Tory division about Europe could, almost unbelievably, lead directly to the election of a hard-left Labour government.

1846 and 1906 also tell us that when a divided Conservative Party leaves office, there can be profound consequences for decades. The division between the Peelites and the protectionists after the repeal of the Corn Laws put the Whigs in office for nearly thirty years, in which time free trade became an almost inviolable policy: division meant that the protectionists were even further away from achieving their objective. In 1906, MPs behaved as though their identities as free traders and tariff reformers were more important than as Unionists, and the consequences of their defeat continue to be felt even now. The Lloyd George ‘People’s Budget’ of 1908 introduced a non-contributory pension system – unlike the contributions-based proposal made by Chamberlain that was rejected by Salisbury and Balfour – and our welfare state to this day remains non-contributory (a fact that is, coincidentally, relevant to the debate about EU nationals claiming benefits in Britain).

A split on such a seminal issue can also cause a realignment in the party system – and realignments, despite the frequent fantasising of politicians and commentators for generations, can occur for better and for worse. Following the repeal of the Corn Laws, the Peelites moved from limited cooperation with the Whigs at first, to coalition with them in the 1850s, to a full-blown merger with them under the name of the Liberals in the 1860s. The 1906 defeat did not cause a grand realignment in the party system but it did drive some liberal Conservatives – among them Winston Churchill – into the Liberal Party. Some Eurosceptic Tories have already defected to UKIP because of the EU, while in the 1990s and 2000s, some pro-European Tories defected to the Liberals and to Labour. The disintegration or fracturing of the Conservative Party is something that the country simply cannot afford.

Arguably, the reason the Peelites went over to the Whigs after 1846 was that the repeal of the Corn Laws symbolised the new dividing line in British politics: between Liberals and Conservatives, rather than Whigs and Tories. Perhaps Tim Montgomerie’s imagined realignment is about to happen anyway, and the referendum will be the catalyst that makes it happen.

But if it is not inevitable that a realignment will take place, history gives us some lessons as to how we might avoid disintegration. And those lessons mostly apply to the conduct of the leaders of the key factions. Peel won the election of 1841 believing he had made no binding pledges on the future of the Corn Laws, while the Party’s county MPs – representing seats very much attached to protection – had mostly made commitments to their voters about maintaining the status quo. When Peel made his move to repeal the Corn Laws, therefore, which he won against the opposition of his own Party and with the support of Whig MPs, he was not just splitting his Party on a single vote on a single issue. He knew he was risking splitting his Party for good.

So ‘constructive ambiguity’, as diplomats call it, can take a leader so far, but in the end, when Peel took a firm and clear position, his coalition of support among Conservatives was brought to an abrupt end. Equally, as the 1906 experience shows, a leader’s ambiguity can cease to be constructive. Balfour (who, perhaps tellingly, liked to say “nothing matters very much and few things matter at all”) allowed tariff reformers and free traders in his Cabinet to believe he was going to support their particular side of the argument. At the general election, he failed to take a lead and offered a compromise, in which the Unionists sought a mandate for attending a Colonial Conference, free of objections to agreeing tariffs, before putting the outcome of the conference to the public in a second election. Unsurprisingly, the electorate saw the proposition for the political sticking plaster it was, and rejected it decisively.

So when a Prime Minister adopts a controversial policy in defiance of his Party, he risks causing a split, but permanent indecision can have the same effect. Arguably, by campaigning to remain inside the EU while offering an In/Out referendum and freedom for his MPs, ministers and activists to campaign to leave, the Prime Minister has negotiated this tricky path successfully.

But how the Prime Minister and his colleagues on both sides of the divide conduct themselves during the campaign is also critical to avoiding rupture. In 1903, when the Duke of Devonshire, a free-trade-supporting Liberal Unionist, told voters not to support a tariff-reforming Unionist candidate in the Lewisham by-election, he started a chain reaction that led to tariff reformers and free traders attacking one another with little regard to Party loyalty. One of the reasons Lord Bentinck and a young Benjamin Disraeli found life difficult as the leaders of the protectionist Conservative rump after 1846 was that their personal attacks on Peel – which continued even after he had been forced from office – were seen as excessive even by their allies. Already, there are signs from advocates of Remain and Leave that the referendum campaign is going to become aggressive, personal and divisive.

That is worrying, because – as these lessons from history show – the next few months represent a moment of danger for the Conservative Party and therefore the country. Personally, I will be voting to leave – for the reasons articulated so brilliantly (and politely) by Michael Gove – but many friends, colleagues, my former boss Theresa May, and the Prime Minister who I’ve worked to get elected twice take a different view. People on both sides of the debate need to accept that we have all taken our positions sincerely and based on what we believe is right for the country. And just as importantly, we need to remember that we have many other shared beliefs that unite us as Conservatives. There remain a great many things we need to do for Britain, whether we are in or out of the European Union. If we behave as though we believe the referendum is more important than every other issue facing the nation, trouble awaits.